How do I remove a render engine in Blender?
There are two good ways to do this:

1) Disabling renderer in the Blender Preferences. This does not fully remove the renderer from Blender, but just disables it so that it does not show.

To do it, go to Blender Preferences (File > Preferences).

Go to Addons, and search "Render", or the name of your renderer. The renderer should show up with the rest of the Blender renderers. Uncheck the blue check next to the Radeon Pro renderer box, or whatever it is called, not (cycles, Eevee, or workbench).

2) Remove it from Blender

If you want to do this then there are two ways. The easiest way I will describe.

To remove it, go to the renderer in Blender Preferences again, and open the tab that describes the addon. If the option is there, select remove. Then remove the renderer from Blender. This should remove it from all of the files, and not be able to be accessed again unless re-installed. 

If it does not have a remove option, do this: 

If in windows, go to the operating file containing Blender, then go to Blender > (blender version) i.e 2.81 > scripts > addons > and search for the addon. Manually delete the file, and none other. Now it should not show up in Blender anymore.